1. Field of the Invention
The present invention relates to an improved technique for the production of zeolites based on silicon oxide and, optionally, oxides of tetravalent elements, and, more especially to the production of silica based MFI zeolites.
The present invention also relates to improved technique for the production of MFI zeolites based on silica and oxides of such tetravalent elements as titanium, germanium, zirconium and/or tin.
2. Description of the Prior Art
The zeolites are known to be crystallized tectosilicates. Their structures include assemblies of TO.sub.4 tetrahedra, which form a three dimensional network by sharing oxygen atoms. In zeolites of the aluminosilicate type, which are the most common, T represents tetravalent silicon and trivalent aluminum. The cavities and channels of molecular dimensions within this network receive the cations which compensate for the charge deficit resulting from the presence of trivalent aluminum in the tetrahedra. Trivalent elements such as gallium and, more rarely, boron or beryllium may be substituted for the aluminum.
The composition can be described by the general formula: x.sub.1 M.sub.1.sup.n1+ ; x.sub.2 M.sub.2.sup.n2+. . [(y.sub.1 T.sub.1 ;y.sub.2 T.sub.2..)0.sub.2 (y.sub.1 +y.sub.2 +..)].sup.x- z.sub.1 A.sub.1 ; z.sub.2 A.sub.2.. wherein the variables within the square brackets represent the composition of the network in T elements, the other variables corresponding to the species in the micropores of the network: M.sub.1, M.sub.2.. are the compensating cations when x is larger than 0 and A.sub.1, A.sub.2 .. represent water, ionic pairs or molecules. Particularly when the T elements are solely tetravalent, there are no negative charges in the network (x=0), and, hence, no compensating cations M.sub.1, M.sub.2...
Each type of zeolite has its own peculiar porous structure. The variation in the dimensions and shapes of the pores from one type to another results in changes in the adsorbent properties thereof.
Only molecules having certain dimensions and shapes can enter into the pores of any particular zeolite. These remarkable characteristics render zeolites particularly suitable for purifying or separating gas or liquid mixtures, for example for separating hydrocarbons by selective adsorption.
The chemical composition, particularly together with the nature of the elements present in the TO.sub.4 tetrahedra and the nature of the exchangeable compensating cations, is also an important factor responsible for the selectivity of adsorption and especially in the catalytic properties of these materials. They are useful as catalysts or catalyst carriers in cracking, reforming or modifying hydrocarbons and in the synthesis of many molecules.
Many natural zeolites exist; these are aluminosilicates, the availability and properties of which do not always meet the requirements for industrial applications. Thus, a wide variety of zeolites essentially of the aluminosilicate type have been synthesized. These include A zeolites (U.S. Pat. No. 2,882,243), X zeolite (U.S. Pat. No. 2,882,244), Y zeolite (U.S. Pat. No. 3,130,007), L zeolite (FR-A-1,224,154), T zeolite (FR-A-1,223,775), ZSM5 zeolite (U.S. Pat. No. 3,702,886), ZSM12 zeolite (U.S. Pat. No. 3,832,449) and ZSM48 zeolite (EP-A-0,015,132).
Zeolites generally crystallize from an aqueous solution. However, their low solubility prevents the formation of substantial quantities of crystals if the reaction medium is only a slightly supersaturated solution, containing the T elements in the form of species which can polycondense to form the network of the zeolites. When the supersaturation of such a solution is increased, an amorphous gel is formed, the solid phase of which contains the majority of the T elements in the form of hydroxides and oxides.
The typical technique for synthesizing zeolites then comprises converting the gel to zeolite crystals by a hydrothermal process, employing a dissolution/recrystallization mechanism. The species constituting the crystals are renewed in the solution by polycondensation, through continuous dissolution of the solid phase of the gel, which serves as a reagent reservoir. This conversion is facilitated by the presence of mobilizing agents, OH.sup.- or F.sup.-, which enable the polycondensable species to be transferred by the solution. The reaction medium also contains structuring agents, which are incorporated in the microporous space of the network during crystallization, thus controlling the construction of the network and assisting to stabilize the structure, through the interactions which are established.
This method of synthesis from a gel nevertheless present certain drawbacks, including the following:
(1) heterogenous composition and texture of the gel, which may effect variations in the composition and nature of the species produced in solution, through dissolution of the gel;
(2) generally incongruent dissolution;
(3) aging during the heating period, which reduces its reactivity;
(4) supersaturation of the solution, which is necessitated by the properties of the gel and which may vary according to the above factors;
(5) practical technical problems, i.e., difficulties in agitating a gel which may be very viscous and in obtaining a homogeneous temperature throughout the reaction medium;
(6) incorporation of particles of gel in the zeolite crystals during their growth, giving rise to heterogeneity in the crystal.